The invention relates to a new type of polypyrrolidone that is suitable for spinning from the melt, and a process for its preparation by anionic polymerization of 2-pyrrolidone.
It has been known since 1951 that polypyrrolidone (also known as nylon-4) can be produced by the polymerization of pyrrolidone in the presence of an anionic catalyst and a polymerization accelerator. Most generally, alkali lactamates have been used as the catalyst. However, in the period between 1951 and 1968, hundreds of different compounds suitable as accelerators have been described in the literature, and by the end of the 1960's the accelerators most generally used were acyl lactams. However, the polymers obtained using these prior catalysts and accelerators tended to decompose when in a molten state, and thus were not stable enough to be spun from the melt. This was a critical deficiency inasmuch as the main use of nylon-4 is as a fiber, and melt spinning is the method of fiber production most generally utilized.
It has subsequently been proposed to use carbon dioxide or sulphur dioxide as an accelerator (see U.S. Pat. Nos. 3,721,652 and 4,105,645). Use of these accelerators in 2-pyrrolidone polymerization yields a nylon-4 having a sufficiently high thermal stability to enable it to be spun continuously from the melt provided that a polymer having a very high molecular weight is prepared. The polymers with a lower molecular weight, e.g. below 80,000, prepared with these accelerators are much less stable and unsuitable for melt-spinning. This stability presumably results from the relatively narrow molecular weight distribution and the very high average molecular weight of these polymers. The accelerators CO.sub.2 and SO.sub.2 always lead to polymers of very high molecular weight, unless a very short polymerization time or an impure monomer are used.
In order to increase the thermal stability of the polymer, all efforts were directed to the production of nylon-4 with a high molecular weight, of, for instance, 200,000 or even over 350,000, which molecular weights correspond to a relative viscosity of from about 15 to 35, measured at 20.degree. C. on a solution of 1 gram of polymer in 100 ml of 96% sulphuric acid. (The term "relative viscosity" as used herein shall be understood to mean the viscosity of a solution of 1 gram of polypyrrolidone in 100 ml of 96% sulphuric acid at 20.degree. C., divided by the viscosity of the solvent at 20.degree. C.) However, when polymers of such high molecular weight were spun, it was found that the relative viscosity of the spun polymer ranged only between about 2.0 and 3.5, which shows that considerable degradation or redistribution of the polymer chains occurs during melt spinning. Consequently, the high molecular weight of the unspun polymer does not result in the formulation of threads of polymer having a high molecular weight.
Furthermore, the high molecular weight of the starting unspun polymer is itself a disadvantage, since the high viscosity interferes with spinning, and the degree of degradation of the polymer to a lower molecular weight is strongly influenced by the conditions during spinning. Thus, small fluctuations in the spinning conditions, particularly small variation in the temperature of the polymer melt, may give rise to uncontrolled variations in the molecular weight of the spun product.
It is indicated in the literature that it might be possible to overcome this problem by subjecting the polymer prior to spinning, to a treatment such as a thermal treatment, which considerably reduces the molecular weight from, for example, 475,000 down to 60,000. This pretreated polymer can then be spun, preferably in the presence of a stabilizer or spinning agent. Such pretreatment tends however to broaden the molecular weight distribution.